The present invention relates to a distance-measuring device and to a method for determining a distance.
Conventional distance-measuring devices are generally employed for detection of the position of the piston in fluidic linear drives and pneumatic or hydraulic cylinders respectively. Said detection in cylinders can take place both discretely, that is at discrete points, as well as continuously during operation.
As a rule, a discrete detection of the position of the piston is used to give feedback of a completed piston-cycle to a control device (e.g. SPS), for example in order to induce another cycle or stage.
For this purpose, one mainly uses sensors sensitive to magnetic fields or sensor devices that detect the magnetic field of a permanent magnet situated on the piston of the cylinder. The sensors are mounted on the outside of the cylindrical pipe of the cylinder. As soon as the piston moves into the measuring range of such a sensor, the latter detects the presence of the piston through the cylindrical pipe. This requires the use of non-ferromagnetic materials, which constrains both the design features and the range of potential applications of the drive.
The detection of other positions of the piston requires the mechanical adjustment of the sensor. For each additional position to be detected another sensor must be installed, which inevitably leads to additional costs for material, assembly, installation, and adjustment.
Furthermore, externally installed sensors require extra space for installation. To ensure accessibility and robustness of the sensor, additional effort in design and construction are necessary.
This type of sensor is predominantly designed as one sensitive to magnetic fields and is commonly known as a Reed-switch, magnetoresistive (MR), giant magnetoresistive (GMR), Hall-switch, or magnet-inductive approximating switch.
The detection of a magnetic field requires adjustment of the magnet to the sensor or the sensor-device. Moreover, this method of measuring restricts the potential applications due to disrupting static and dynamic magnetic fields (EMV, field of a nearby cylinder) as well as due to the thermal reactions of the sensor.
Systems used for the continuous measuring of piston-positions generally operate potentiometrically, according to the LVDT-principle (Linear Variable Differential Transformer), or according to the ultrasound-principle. With such systems, the position of the piston is expressed continuously and most often as an analogous voltage-signal. In addition to those systems there are also methods of incremental distance-measurement. The latter may, for example, put codes on the piston rod and are therefore only capable of measuring relative distances.
The continuous as well as the discrete detection of the piston position often cannot be integrated into the cylinder. Even where this is possible, the procedure leads to considerable constructional and financial expenditure. This is due to the fact that the measuring range of all above-mentioned conventional sensor mechanisms is too limited and therefore requires constant adjustment to the length of the respective cylinder.
It is the task of the present invention to develop a distance-measuring device and a method for determining a distance capable of overcoming the disadvantages described above. The device and method shall allow for the continuous and discrete measuring of distances, they shall be easy to operate, and they shall be fit for a wide variety of applications.
This invention provides a distance-measuring device and a method to determine distances, in which a sensor arrangement includes a coupling probe. This probe measures a specific distance, for example in a cylinder, through the emission and reception of (sound) waves, for example by integrating the coupling probe into the cylinder. The integration of this coupling probe into the cylinder makes it possible for the distance measuring device to be built small, and requires little or no alteration to the device. Because there is no need for a mounting for external sensor devices, the entire setup of the proposed distance-measuring device can have a clean, level design, and the external appearance is not altered. Installation costs are decreased in the proposed device, because the prefabricated cylinder has only a connecting cable for control and data acquisition. Furthermore, this allows for the separation of the sensor device from the control and evaluation electronics. The latter can be operated externally and remotely from the distance-measuring device that controls the coupling-probe. Usage in a high-temperature environment is possible, in particular between the range of 300 and 1000 degrees Celsius, without problems. According to the proposed method, the length of a circuit is measured up until a short-circuit device, which where appropriate can be adjusted. The transmission signal of the intended distance-measuring device is transmitted into the circuit, and is reflected by a specific section of the circuit, preferably a short-circuit device. Thus the distance between the introductory point and a specific section of the circuit is measured. The distance is measured by determining the transit-time of the signal.
When using a frequency-modulated transmission signal, the distance is calculated using the following formula:
Distance=nxc3x97(C/2)xc3x97(frequency deviation)
where n=1,2,3 . . .
and C=speed of light
This method of measuring the distance achieves a rate of accuracy of a half-wavelength of the transmission signal. A procedure to measure the distance carried out in this manner, measuring the distance to a specific section of the circuit, is referred to herein as a xe2x80x9csearch process.xe2x80x9d
Further advantageous designs of the proposed device are the subject of the dependent claims. For example if a coupling probe allows a magnetic or electric coupling, and/or a slot coupling, then the cylinder functions as a wave-guide and/or a coaxial lead.
Depending on the desired mode, the coupling probe emits an electromagnetic wave in high-frequency range, preferably between 10 MHz and 25 GHz, into the cylinder, in order to provide the best possible signal processing. Depending on the dimensions and/or size of the cylinder, frequencies in the lower range can be used, after which the next-highest mode is diffusible. Practice shows that, particularly with regard to a piston, single-mode diffusion is advantageous, preferably in TEM-mode. In this mode the TE11-field-type is promoted as the next-highest mode.
The resulting frequency ranges for cylinders with a diameter D and the piston-rod with diameter d, where D=10 mm and d=4 mm, are approximately 14 GHz for a frequency in the lower range of the TE11 Mode, and where D=25 mm and d=10 mm, approximately 5.5 GHz for a frequency in the lower range of the TE11 Mode.
It should be noted in this context, that in a piston-cylinder, the TE11 Mode is suppressed by the straight symmetry of the axis of the field-excitation/stimulation as well as of the field-space. The width of the frequency range, in which next-higher field types are diffusible, can be almost doubled due to this axial-symmetry. In this example, the next-higher diffusible mode is the TE21 Mode. However, it should be noted that in cylinders with a straight piston rod, apart from field-types of the coaxial lead, field-types in the circular waveguide exist as well. For all cylinders, the frequency range of this field type in a circular waveguide is higher than the respective frequency range of the field type in cylindrical coaxial lead. If, for example, one uses an operating frequency that allows only for the diffusion of the TEM field type in the coaxial lead, no other field types of the waveguide are diffusible in the entire cylinder.
If, the coupling is singular, therefore non-symmetrical, the TE11 Mode is diffusible in a coaxial cylinder. If, however, multiple introductory points with axially-symmetric couplings are used, the TE11 Mode is suppressed in a coaxial cylinder. In this case, if two coupling probes transposed at 180 degrees are used, both probes are supplied through an introductory point by splitting the HF-signal through a 3 dB-power coupler or a power divider, e.g. Wilkinson. For four coupling probes in 90-degree transposition two 3 dB-couplers are used; eight coupling probes in 45-degree transposition require four 3 dB-couplers. The advantage of axially-symmetric introduction consists in the suppression of the next-higher mode, which allows for the use of a higher transmission frequency. This higher transmission frequency and the resulting greater bandwidth achieve increased accuracy in measuring.
Creating a measuring-device that contains a matching network, preferably a high-frequency network, is favorable because it increases the frequency range of the probe. This makes possible the emission and reception of a frequency-modulated transmission signal. Such a matching network is a prerequisite for determining a distance through the xe2x80x9csearch processxe2x80x9d and the corresponding search-algorithm most accurately. Preferably, both the coupling probe and the matching network should be designed as passive power structures in the form of a thin layer of gold (e.g. 15 mm), preferably produced by galvanization. For practical reasons one might favor a coupling that is singular, non-symmetric to the axis and thus forfeit the advantages of a symmetric coupling, namely a higher transmission frequency and a greater accuracy of measuring. This makes it possible to work with an identical probe for almost all commonly used types of circuits, in particular all sizes of piston-cylinders.
The symmetrical coupling with several coupling probes has another advantage: the transmitter and the receiver can be connected to separate antennas. To achieve this when using, for example, four coupling probes, one should use two probes situated opposite from one another for each the transmission and the reception. If the transmission branch and the reception branch are not separated both the coupling probe and the circuit of the transmitter are used for the receiver until separated by the link-up. The coupling probe includes an insertion damper. Consequently, a part of the transmission signal is reflected at the coupling probe and enters the receiver. Inside the receiver, the reflected part of the transmission signal overlaps with the actual reception signal and reduces the accuracy of the measurement. Separating the transmission branch from the reception branch at the antenna will avoid this problem.
The separation of transmission antenna and reception antenna has another advantage: different designs can be used as transmission- or reception antenna, e.g. electric or magnetic probes or a slot coupling. Combinations of those designs are also possible. This allows for a direct feedback of the transmission signal into the receiver and for an improvement of the quality of the signal.
If, one uses the high-frequency electronics of the sensor device (split up into a transmission branch and a reception branch), the reception branch of which consists of a mixer and/or at least four high-frequency diodes, then it is possible to detect the direction of the movement of a predetermined part of the circuit and to clearly determine a distance change of this part.
If a closed control loop is implemented a frequency, e.g. of the Voltage Controlled Oscillator (VCO), which, for example, has been diverted from the transmission branch, cannot be used directly as the final quantity. Instead, this quantity can be used in frequency and phase control. This method makes possible a direct, simplified and particularly quick processing and interpretation of the signals to determine the distance.
This dynamic frequency-regulation can be controlled through a Phase-Lock-Loop (PLL), which consists of at least one frequency divider, one phase discriminator and one low-pass filter. In this case, the index-frequency is predetermined by a Direct-Digital-Synthesizer.
If the reception branch contains an IQ detector (In-Phase/Quadrature Detector), this special design makes possible the detection of the direction of a movement of a predetermined part of the circuit.
Another advantageous, simplified design for the Search Mode is created when the frequency deviation of the oscillator and the length of the delay line are chosen to correspond with a specific, predetermined distance between the coupling probe and a section of the circuit, e.g. in a piston. This means that a synchronization point within the circuit is predetermined. When the synchronization point is crossed by a piston rod, for example, then the transmitter synchronizes immediately, it switches to the Track-Mode and takes over the highly dynamic detection of the piston position.
Furthermore, if a synchronization point is chosen relatively remote from the coupling probe, the procedure described above offers the advantage that the delay-line can be kept short, e.g. as a printed lead on the backside of the coupling probe. Moreover, the frequency deviation can be kept small.
The procedure has another advantage: choosing a point of synchronization relatively remote from the coupling probe makes it possible to keep the delay line short (e.g. applied to the backside of the coupling probe), and to keep the frequency deviation low.
Further advantageous designs are subject of the remaining dependent claims.